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The Electron launch vehicle first stage has nine Rutherford engines In addition to adapting the single-engine-out-friendly nine engine configuration from the Falcon-9, the rocket carries a significant mass of batteries in order to power two electric motor-driven propellant pumps for fuel and oxidizer.

For a given amount of work (Joules) the lower energy density (Joules per kilogram) of electrical batteries means they are something like 50 times heavier than the LOX/RP-1 needed to do the same work.

The tradeoff is simplicity versus newness of technology and battery mass. Newness did not seem to be a problem (highly reliable electric motor driven pumps in hostile environments are a mature technology on Earth) and the battery mass can be at least party offset by dropping spent batteries ("battery staging").

Question: Does electric fuel pumping scale well for rockets? Is the Electron at a "sweet spot" in terms of size, or would smaller and (much) larger rockets see similar tradeoffs (qualitatively and quantitatively) if going the electric fuel pump route?


Related background:

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    $\begingroup$ Just a guess: the power requirements to pump the fuel needed for something like an F1 or even just a Raptor likely make it infeasible. Neutron is going to have turbopump-fed engines, so at least Rocket Lab itself seems to have doubts. The allure of electric pumps is the simple startup (no "chicken and egg" problem), throttle ability down to 0 (for the pump side at least, obviously combustion must still be sustained), and the ability to completely drain the tanks. $\endgroup$ Sep 6, 2021 at 11:05
  • $\begingroup$ @JörgWMittag naively I would think that it would scale up at least quite nicely, i.e. linearly; 10x more massive rocket with 10x the fuel needs 10x bigger pumps and 10x bigger batteries. I can't think of any reason why it wouldn't. What makes you think "the power requirements to pump the fuel needed for something like an F1 or even just a Raptor likely make it infeasible"? What am I missing? $\endgroup$
    – uhoh
    Sep 6, 2021 at 22:40
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    $\begingroup$ @JörgWMittag I am having trouble seeing how this wouldn't scale. Why would a rocket that's 10x heavier needing 10x thrust to follow the same launch trajectory not simply need 10x the battery mass and 10x the motor mass. Basically I won't believe some isolated envelope back numbers. It works nicely for the Electron. Unless someone can point out why it wouldn't scale for larger rockets, I'm going to believe that it does. $\endgroup$
    – uhoh
    Sep 7, 2021 at 11:20
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    $\begingroup$ I can image a two-stage design: battery powers a electric pump that feeds the turbine, and the turbine pumps the main engine. This way you more or less get the performance of a closed-cycle with the complexity of an open-cycle. $\endgroup$ Sep 8, 2021 at 3:48
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    $\begingroup$ The scaling might be in an area different from the one you expect.. The cost of developing and maintaining a fuel based turbo pump might be worthy for large payloads but not for small ones. $\endgroup$
    – Antzi
    Jan 25 at 5:49

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No.

First of all, I don't think electron is at a "sweet spot". In fact, I'd guess that the electric system results in--at best--a neutral result in terms of mass budget, but more likely, a negative one. If anything, it is at a "this is barely possible" spot.

Specifically, when asked about this design decision Peter Beck says things like:

...beauty with an electric turbopump is that it takes that really complicated problem and turns it into software...

...eliminates the complex valves and other plumbing required to use hot gas to turn turbomachinery, boosting efficiency from 50% for a typical gas generator cycle to 95%.

Nowhere can I find references to this design decision being optimal from a mass perspective: everywhere it's always about increasing simplicity, reducing parts, and making it easier to design alongside other advantages like fast and deep throttle control.

Wikipedia summarizes this as:

In comparison to turbo-pumped rocket cycles such as staged combustion and gas generator, an electric cycle engine has potentially worse performance due to the added mass of batteries, but may have lower development and manufacturing costs due its mechanical simplicity, its lack of high temperature turbomachinery, and its easier controllability.

The citation from this comes from a a 2010 paper on this very topic. The paper, titled "ELECTRIC FEED SYSTEMS FOR LIQUID PROPELLANT ROCKET ENGINES." has many visualizations, which I suggest you check out.

While we don't know exactly at which chamber pressure the Rutherford engine operates at, nor are the scenarios pictured an exact match for Electron (I don't think they are discarding empty batteries in their modeling), the analysis supports this conclusion.

Specifically, the energy density of LOX/RP1 is just so high compared to any conventional battery technology that even at an inefficiency of 50%, it still smokes the far more efficient batteries (ballpark 3500 Wh/Kg for RP1/LOX and like 200 Wh/Kg for batteries).

Conclusion:

Electrical pump systems have many advantages that make them an appealing design decision, engineering simplicity and ease-of-control being chief among them. Scalable and efficient from a mass-perspective--they are not. The enormous energy density of traditional liquid rocket propellants dwarfs that of battery packs by so much that even an extremely inefficient pump can still trounce an electric pump.

That said, there may be an interesting "hybrid" approach here that I haven't seen any references of, where the engine is electrically pumped, however the power is supplied from an auxiliary engine that's purely an electrical generator as opposed to batteries. This might allow the advantages of both to combine: a mechanically simple system with great controllability, but that does away with heavy battery packs and has an auxiliary generator instead. So if anyone needs a topic for a Master's thesis...

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    $\begingroup$ Shuttle ran into similar problems when they tried to replace the hydrazine-powered hydraulic pumps with battery-powered electric ones. For safety and maintenance and probably every other consideration the electric ones were superior, but they just could not get the weight down to a practical level. $\endgroup$ Jan 27 at 14:18

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